Fiber-optical communications systems have revolutionized the wire-dependent data transmission over large distances within a few years. In connection with directional radio installations, which had been dominant up to that time, systems already in service today can be considered to be superior in every respect in view of the available bandwidth. Only mobile communications are able to profit indirectly from this advance by means of efficient fixed networks, since cellular networks also need to utilize narrow-band and trouble-prone radio on a portion of the transmission path. In connection with the transmission via or between satellites, large distances still need to be overcome, which absolutely requires large transmission outputs and antennas, which in turn runs counter to the desire for systems as compact and as light as possible for space travel. For this reason efforts were undertaken fairly soon after the triumphal march of the fiber-optical communication technology to also use its advantages for optical communications in free space by means of suitable systems.
New factors in the fiber-optical communications technology, inherent in the system, have shown themselves to be limiting, in particular in connection with bridging great distances, especially the dispersion in the dielectric wave guide used for transmission, and various non-linear effects of its material. Optical communication in free space again meant the return of old limiting effects of radio technology and wire-dependent communications. Here, the loss of signal output on the transmission path and the effects of foreign signals dominated again. However, in fiber-optical communications the extreme limits of the energy of a symbol used for transmission are not expressed by the terms describing the phenomenon of thermal noise, but by means of photons per bit.
For example, at an error quotient of 1/1,000,000,000, 10.5 photons per bit are inherently required for the assured transmission of data by means of intensity modulation (J. S. Senior, "Optical Fiber Communications, Principles and Practice", second edition, Prentice Hall, pp. 469 to 471).
Better results can be achieved with pulse-position modulation, as well as various coherent techniques, in particular methods with homodyne transmission. The best realized results were obtained by means of homodyne superimposition (less than 30 photons per bit). Since there is a clear requirement for low energy consumption for space-based systems, an optical system for data transmission between distant geostationary satellites should transmit and receive light waves by the largest possible and very accurately aligned aperture. This, in turn, can only be realized, starting at a defined size and while maintaining a low weight, in the form of a reflecting telescope. Reflecting telescopes in the so-called coaxial form are known in numerous designs, the systems in accordance with Gregory, Cassegrain and Schmidt should be mentioned (Eugene Hecht, "Optics", second edition, Addison-Wesley Publishing Company, Reading, MA, USA, pp. 197,198).
Common to all of them is the system-related disadvantage of the partial central covering of the aperture by the collecting mirrors and their suspension devices. In this case a compromise between mechanical sturdiness and losses because of covering of the aperture must be found.
Generally, an additional screen is required, which prevents the reflection of scattered light, which is encouraged by the collecting mirror and its suspension, in the direction of the light to be received. The simultaneous use of such a telescope for radiating a light wave as well as for receiving an oppositely entering light wave generally results in significant disadvantages, since the said collecting mirror and its suspension reflects a portion of the high output transmitted light in the direction of the simultaneously entering light wave and results in interferences because of superimposition. Accepting great losses regarding the imaging quality, this problem can be bypassed by the use of an oblique reflecting telescope proposed by Kutter. However, the mentioned imaging errors result in the waste of valuable transmission output.